The present invention relates to a nitrile polymer composition, a molded article thereof, and a process for producing the composition. More particularly, the present invention relates to a novel nitrile polymer composition having an excellent melt-flow property and an excellent gas barrier property, a process for producing the same, and a molded article of the composition.
Nitrile resins containing an acrylonitrile or methacrylonitrile polymeric unit in an amount of 50% or more show excellent gas barrier property (high gas impermeability) owing to the inter-molecular bond unique to nitrile group. The nitrile resins are also excellent in chemical resistance (e.g. resistances to acids, alkalis, organic solvents) and mechanical properties (e.g. flexural modulus, tensile strength, creep resistance) and accordingly are balanced in properties. Because of these many advantageous properties, the nitrile resins are in wide use in food packaging films, sheets and materials for containers.
In recent years, a method for food storage not using any additive (e.g. antioxidant) has seen progress. Along with this movement, it is desired to develop a food packaging material having a higher gas barrier property. However, the development of such a food packaging material is not easy technically.
For example, the gas barrier property of acrylonitrile resin becomes generally higher as the acrylonitrile content in the resin becomes higher; meanwhile, acrylonitrile resins, when heated to temperatures higher than 200.degree. C., cause cyclization owing to the intra-molecular condensation and resultantly are colored and infusibilized. Therefore, an acrylonitrile resin having an acrylonitrile content of 85% or more is virtually impossible to melt-mold.
Japanese Patent Publication No. 25005/1971 discloses a process for producing a thermoplastic nitrile polymer having low gas permeability. In this process, 100 parts by weight of a mixture consisting of (A) at least 70% by weight of an alpha,beta-olefinically unsaturated mononitrile such as acrylonitrile or the like and (B) 30% by weight or less of an olefinically unsaturated carboxylic acid ester such as methyl acrylate or the like is polymerized at 0.degree.-100.degree. C. in the presence of (C) 1-20 parts by weight of a copolymer of an olefinically unsaturated nitrile and a conjugated diene selected from the group consisting of butadiene and isoprene, in an aqueous medium in the presence of an emulsifier and a radical polymerization initiator in the absence of molecular oxygen. The nitrile polymer produced by the process contains a copolymer of a conjugated diene and an olefinically unsaturated nitrile, as mentioned above.
U.S. Pat. No. 3,742,092 discloses a process for producing a rubber-modified methacrylonitrile homo or copolymer having impact resistance, by polymerizing at least 80 parts by weight of methacrylonitrile and 0-20 parts by weight of a monomer selected from methyl acrylate, methyl methacrylate and styrene, in the presence of 1-40 parts by weight of a performed diene rubber, 5-160 parts by weight of a seed polymer and a radical initiator. The seed polymer is a polymethacrylonitrile, a copolymer of methacrylonitrile and up to 20% by weight of other monovinyl monomer, a polystyrene, a polymethyl acrylate, a polymethyl methacrylate, a polyacrylonitrile, or a copolymer of acrylonitrile and other monovinyl monomer.
The above rubber-modified methacrylonitrile homo- or copolymer is also characterized by containing a diene rubber as mentioned above.
U.S. Pat. No. 3,732,336 discloses a process for producing a methacrylonitrile homo- or copolymer by emulsion-polymerizing at least 80 parts by weight of methacrylonitrile and 0-20 parts by weight of other monovinyl monomer in the presence of 5-160 parts by weight of a seed polymer and a radical initiator. As the seed polymer, there is used the same seed polymer as described in the above U.S. Pat. No. 3,742,092. The U.S. Pat. No. 3,732,336 describes that the process can give an improved polymerization rate, but the document gives no specific data regarding the production of a particulate polymer in which the seed polymer is a high-nitrile copolymer and the shell layer polymer is a low-nitrile copolymer.
Japanese Patent Application Kokai (Laid-Open) No. 69814/1986 proposes an acrylonitrile polymer whose molecular weight is made smaller (reduced viscosity=1.0 or less) to enable melt molding, based on a general knowledge that acrylonitrile polymers generally have low melt moldability because of the high molecular weight.
Further, Japanese Patent Application Kokai (Laid-Open)No. 236210/1989 discloses, as a melt-moldable acrylonitrile polymer, an acrylonitrile polymer which has a reduced viscosity of 0.1-1.0 and an acetone-soluble content of 5-20% by weight.
Japanese Patent Application Kokai (Laid-Open) No. 62848/1973 discloses an acrylonitrile copolymer-graft rubber composition having low CO.sub.2 permeability.
Japanese Patent Publication No. 21331/1984 discloses a polymer composition obtained by graft-polymerizing 80-95 parts by weight of a monomer such as acrylonitrile or the like in the presence of 5-20 parts by weight of a rubber, 100 parts by weight of which composition contains 2-40 parts by weight of a resin of the monomer grafted onto the rubber and 40-93 parts by weight of a matrix resin of the monomer.
Japanese Patent Application Kokai (Laid-Open) No. 167318/1989 discloses a high-nitrile resin obtained by grafting 60-99 parts by weight of a monomer mixture consisting of an unsaturated nitrile and an aromatic vinyl compound, onto 1-40 parts by weight of a diene synthetic rubber.
All of these proposed polymers and processes are characterized by using or producing a polymer containing a resin of high unsaturated nitrile content. Japanese Patent Publication No. 25005/1971 and Japanese Patent Application Kokai (Laid-Open) No. 62848/1973 are intended to effect graft polymerization in the presence of a rubber component to impart impact resistance and gas barrier property; U.S. Pat. No. 3,742,092 is intended to effect graft polymerization in the presence of a rubber component and a seed polymer to impart impact resistance and achieve improved polymerization rate; and U.S. Pat. No. 3,732,336 is intended to achieve improved polymerization rate by adding a seed polymer. All of these processes, however, are unable to obtain a resin composition well balanced in melt-flow property, gas barrier property and mechanical properties. Japanese Patent Publication No. 21331/1984 and Japanese Patent Application Kokai (Laid-Open) No. 167318/1989 are intended to effect two-stage polymerization to allow the polymer obtained by applying graft polymerization to a rubber component and the matrix polymer to have different compositions and thereby to impart solvent resistance, but the resulting polymers are insufficiently balanced in melt-flow property and gas barrier property. Japanese Patent Application Kokai (Laid-Open) 69814/1986 and Japanese Patent Application Kokai (Laid-Open) No. 236210/1989 are intended to obtain a polymer of reduced molecular weight to impart melt-flow property, but the resulting polymers inevitably have low mechanical properties.
An object of the present invention is to provide a rubber-containing high-nitrile polymer composition having excellent melt-flow property, gas barrier property and mechanical properties and a process for producing the composition.
Another object of the present invention is to provide a high-nitrile polymer composition melt-moldable to a film, a sheet, a bottle, a fiber, etc. and a process for producing the composition.
Still another object of the present invention is to provide a high-nitrile polymer molded article, having the properties of the above composition.
Other objects and advantages of the present invention will become apparent from the following description.